Since very ancient times, going back at least to the Shang Dynasty in China, the so-called "lost-wax" process has been employed for the manufacture of castings. Essentially, this process comprises forming a pattern of wax or blends of various suitable waxes and resins. This finished pattern is then invested with a suitable medium, such as a ceramic or refractory slurry, which is then solidified and becomes a mold around the wax pattern. The pattern material or more commonly the wax is then removed from the mold by melting it and/or by burning, and a molten metal is poured into the now empty mold to produce the finished part. Further description of investment casting are found in U.S. Pat. Nos. 3,263,286 and 3,667,979, as well as in the Investment Casting Handbook of the Investment Casting Institute, 1968.
Such a process has many obvious advantages for making parts, especially intricate parts, which cannot be made, for example, by machining. However, the properties of the casting waxes are extremely important for the production of such parts in modern day usage of this process.
A summary of some of the requisite physical properties of such pattern materials is found in the British publication entitled "PATTERN MATERIALS AND THEIR USE IN INVESTMENT CASTING" of the Pattern-Making Committee of the B.I.C.T.A.
One of the major drawbacks in the use of investment casting wax compositions is that conventionally the wax must be injected at temperatures somewhat above room temperature, i.e. 105.degree. F. to 190.degree. F. Waxes are non-Newtonian fluids so that when injected under pressure, they are subjected to shearing forces causing them to become somewhat more fluid. It is not necessary to heat waxes until they are completely liquid in order to completely fill the interstices of a die; nevertheless, they must be heated to some extent. Thus, when injected into a complicated die, thin wax sections cool relatively quickly, reproducing that particular section with a high degree of dimensional accuracy, but the wax in heavier sections will shrink considerably more. Additionally, because of these different rates of cooling and shrinkage, considerable stress can be imposed on the wax pattern, and when removed from the die, the pattern may readily distort in order to relieve the strain.
To a limited extent, such dimensional problems can be compensated for by retooling of the master die. Such retooling is a complicated procedure at best, and in addition, such a tailoring of a die is costly and is not completely reliable. Moreover, heating up of the die with repeated injections, large ambient temperature variations and the like, all further combine to produce a considerable loss of precision in the mass reproduction of pattern dimensions.
This problem in investment casting caused by distortion of the patterns due to shrinkage has long troubled the art. Solution of this problem has been attempted by inclusion of various filler materials in the investment casting wax composition. While this feature has met with some degree of success, it has also introduced some new problems as well as leaving other problems unsolved.
For one thing, the wax composition must be safe to handle as a solid and must be capable of convenient melting or burning out of a mold. It is also essential that the wax composition have a low ash content; that is, about 0.02% or less. It is also necessary that such wax compositions be of sufficient strength and be hard enough at room temperature, so that the patterns be self-supporting and can be handled without damage.
Among the various materials that have been suggested as useful filler for such investment casting wax compositions but which have not met all the desirable physical properties for pattern making or have resulted in new problems there may be mentioned that polystyrene beads can be used as suggested by U.S. Pat. No. 3,465,808. However, during autoclave removal of the wax, the wax melts out first leaving a polystyrene residue that tends to tear the ceramic mold wall.
Moreover, there is an excessive breakage of these ceramic shells, such that they become unsuitable for use as molds for receiving molten metal. This recent occurrence of excessive shell mold breakage occurs during the dewaxing of ceramic shells.
Accordingly, it has been further suggested that an improved type of polystyrene beads that may be employed are beads of a cross-linked styrene-divinylbenzene copolymer. Such material appears to be highly desirable as a filler for investment casting waxes. This cross-linked material is a neutral pH filler and thus will not attack either acid or alkaline sensitive ceramic material in shells for high quality metal castings.
Cross-linked beads of said copolymer have been available for a number of years and yet a number of serious drawbacks are present with such commercially available cross-linked styrene-divinylbenzene copolymer beads. It is to be understood that to be especially suitable and useful for use as a filler in wax compositions for investment casting, the cross-linked material must not be of too large a particle size. If essentially all or at least about 90% of the beads will not pass through a 100 mesh screen one obtains an "orange peel" effect in the casting process and thus the beads are not suitable for investment casting of intricate and/or complex parts.
This size requirement has lead to a significant problem. While cross-linked styrene-divinylbenzene copolymer beads of such desirable size, i.e. essentially all passing through a 100 mesh screen, have been available from Cole Polymers Ltd. in the United Kingdom, the cost of such copolymer beads is only available at a prohibitively high cost of over $3/lb, thus, economically, use in investment casting is precluded. In the United States copolymer beads of such small size are only available in an extremely limited quantity from Dow Chemical Company and at a cost of $12/lb. The only copolymer beads available, at a cost which permits their use as a filler for investment casting, are beads which are too large in size, namely 18 to 100 mesh beads.
Therefore, it is highly desirable that a cross-linked styrene-divinylbenzene copolymer be available at low enough cost, i.e. about $1.00/lb or less, and of a suitable size as stated hereinbefore to enable its use as a filler in investment casting waxes.